Circuit breakers currently used to protect residential environments are described in U.S. Pat. No. 4,513,268 entitled "Automated Q-Line Circuit Breaker". In the aforementioned patent, trip units in the form of combined thermal and electromagnetic sensors are used to determine the presence of overcurrent conditions and to release an operating mechanism to separate the circuit breaker contacts.
To provide ground fault protection facility to such residential circuit breakers, a ground fault module such as described in U.S. Pat. No. 4,667,263 entitled "Ground Fault Module for Ground Fault Circuit Breaker" is attached to the circuit breaker and interacts with the circuit breaker operating mechanism to interrupt circuit current upon occurrence of a ground fault.
As described in the aforementioned US patents, current transformers are required to sense the circuit current in one of two ways. In one approach a first current transformer senses the circuit line current and a second transformer senses the circuit neutral current and a comparison circuit is used to determine whether there is a remainder current as indicative of ground fault. Another approach uses a summing transformer to surround both the line and neutral conductors and determines the presence of a ground fault when the resultant current is above a predetermined value.
The size of the current transformers are governed by the corresponding size of the magnetic cores to insure that the cores do not become saturated upon occurrence of a large overcurrent. The size of the current transformers are further increased in proportion to the ampere rating of the protected circuit. U.S. patent application Ser. No. 08/726,896 filed Oct. 4, 1996 entitled "Circuit Breaker with Arcing Fault Detection Module" describes the use of a current transformer of reduced size for sensing arcing faults in residential applications.
In certain so-called "industrial-rated" circuit breakers, the current transformers are enclosed within the same enclosure that houses the circuit breaker operating mechanism such the available space for containing the transformers is somewhat limited.
In view of the economic advantage with circuit breakers employing thermal-magnetic trip units for residential applications, it would be desirable to incorporate ground fault protection within such circuit breakers without incurring the costs associated with requisite current transformers for sensing circuit current.
One purpose of the invention is to describe a ground fault detection module that can be attached to a thermal-magnetic circuit breaker for sensing ground faults and interrupting the protected circuit immediately upon detection thereof.
A further purpose of the invention is to allow efficient current sensing within the ground fault module without requiring the use of current transformers, per se.